The present invention generally relates to a magnetic memory element suitably usable in a high-density magnetic memory such as magnetic random access memory (MRAM).
Recently, the application of magnetic tunnel junction (MTJ) elements to playback magnetic heads for hard disk drives (HDDs) and to magnetic memories has been considered and discussed because the MTJ elements provides a higher output, compared with conventional anisotropic magnetoresistive (AMR) elements and giant magnetoresistive (GMR) elements.
In particular, magnetic memories, which are solid state memories having no operating parts like semiconductor memories, are more useful than the semiconductor memories because of the following characteristics of their own: the information stored therein is not lost even if electric power is disconnected; the number of repetitive rewrites is infinite, namely, an infinite endurance is provided; there is no risk of destroying the recorded contents even if exposed to radioactive rays, etc.
FIG. 6 shows the constitution of a conventional MTJ element. Such a constitution is disclosed, for example, in JP-A-9-106514.
The MTJ element in FIG. 6 is constituted of the following stacked layers: an antiferromagnetic layer 41, a ferromagnetic layer 42, an insulating layer 43, and a ferromagnetic layer 44. As the material of the antiferromagnetic layer 41, an alloy such as FeMn, NiMn, PtMn or IrMn is used. As the materials of the ferromagnetic layers 42 and 44, Fe, Co, Ni or an alloy thereof is used. Further, as the insulating layer 43, various oxides and nitrides are being studied, and it is known that the highest magnetoresistance (MR) ratio is obtained when using an Al2O3 film.
In addition to this, there has been proposed an MTJ element without the antiferromagnetic layer 41, which utilizes a difference in coercive force between the ferromagnetic layers 42 and 44.
FIG. 7 shows the principle of operation of the MTJ element having the constitution shown in FIG. 6 where the MTJ element is used for a magnetic memory. The directions in which the ferromagnetic layers are magnetized are indicated by arrows.
The magnetizations of both the ferromagnetic layers 42 and 44 are parallel to the film surface and have effective uniaxial magnetic anisotropy such that the magnetizations of these layers are parallel or antiparallel to each other. The magnetization of the ferromagnetic layer 42 is substantially fixed in one direction by the exchange coupling with the antiferromagnetic layer 41, and a recorded content is represented by the direction of magnetization of the ferromagnetic layer 44.
The resistance of the MTJ element differs depending on whether the magnetization of the ferromagnetic layer 44 to serve as a memory layer is parallel or antiparallel to the direction of magnetization of the ferromagnetic layer 42. Utilizing the difference in resistance, information is read from the MTJ element by detecting its resistance value. On the other hand, information is written to the MTJ element by changing the direction of magnetization in the ferromagnetic layer 44 using a magnetic field generated by electric current lines placed in the vicinity of the MTJ element.
In using the MTJ elements as memory cells in a magnetic memory, it is necessary to reduce influences of thermal noise so as to enable a sense amplifier to perform the sensing. To this end, the MTJ element should have a resistance reduced to some degree. To reduce the resistance of the MTJ element, the insulating film of Al2O3 must be formed as thin as 1 nm or less.
With such a thin Al2O3 the MTJ element can have a reduced resistance. However, the magnetoresistance ratio also tends to deteriorate. This may be ascribable mainly to a method of forming the Al2O3 film. Specifically, in a process of oxidizing a thin Al film of a thickness of 1 nm or less in an oxidizing plasma ambient, because active oxygen is used in the form of ions or radicals, it is difficult to selectively oxidize the thin Al film only. Therefore, if one tries to oxidize the Al film sufficiently, there is a strong possibility that a ferromagnetic layer surface touching the Al barrier layer is also oxidized though partially. To the contrary, if one tries to avoid such oxidation of a ferromagnetic layer, oxidization of the Al film may result insufficient.
In addition, it is very difficult to form a very thin Al2O3 of 1 nm or less without any pinholes. Existence of pinholes in the Al2O3 insulating film would cause reduction of yields in an application requiring a multiplicity of MTJ elements such as in a magnetic memory.
In view of the above problems, it is an object of the present invention to provide a magnetic memory element that has a resistance and a magnetoresistance ratio which meet requirements of a magnetic memory, but yet is free from reduction of yields due to pinholes, and also to provide a magnetic memory using such magnetic memory elements as memory cells.
In order to accomplish the above object, a magnetic memory element according to the present invention comprises:
a first ferromagnetic layer;
a second ferromagnetic layer; and
a non-magnetic layer disposed between the first and second ferromagnetic layers, the non-magnetic layer having an electrical characteristic that is changeable depending on an external magnetic field applied to the non-magnetic layer.
According to the present invention, because the non-magnetic layer is not required to be an insulative film, it is possible to reduce the resistance of the magnetic memory element, without deteriorating its characteristics. Furthermore, no film thickness restriction, such as being 1 nm or less, is not imposed upon the non-magnetic layer not of an insulative material such as Al2O3. Thus, it is possible to form the non-magnetic layer without pinholes.
The non-magnetic layer may be formed of a III-V group compound. As the III-V compound, III-V compound semiconductors such as InSb, InAs, GaAs, InAsP, can be used.
The III-V compound semiconductors generally have a great mobility so that their electrical characteristics are easily changed by the external magnetic fields.
Alternatively, the non-magnetic layer may be constituted of a diode, such as a tunnel diode. Use of a diode for the non-magnetic layer will contribute to the improvement of the characteristics of the magnetic memory element.
As a further alternative, the non-magnetic layer may be formed of a IV group semiconductor, e.g., Si or Ge. In particular, when using Si, there is an advantage that amplifiers and peripheral circuits necessary for a memory are fabricated at the same time.
As a still further alternative, the non-magnetic layer may be formed of a film of MnSb fine particles.
Preferably, the non-magnetic layer may have a thickness of more than 1 nm, but less than 100 nm.
The first and second ferromagnetic layers may have perpendicular magnetizations. Alternatively, they may have horizontal magnetizations.
Use of the magnetic memory element of the present invention enables production of magnetic memories with improved yields.
Other objects, features and advantages of the present invention will be obvious from the following description.